1. Field of the Invention
The invention relates to the art of making radar transparent ceramic windows and,, in particular, to a monolithic microwave radar transparent radome which is strong and erosion resistant.
2. Description of the Prior Art
In various types of hypervelocity missiles and reentry vehicles carrying radar equipment, an antenna is mounted in the nose of the craft and is covered with an appropriate aerodynamic window or radome. The radome must be constructed of material which is strong enough to withstand the aerodynamic forces to which it may be subjected, and yet must be relatively distortion-free and highly transparent to radar energy, particularly at high temperatures. The missile will be traveling at supersonic speeds and such vehicles may be subjected to even higher temperatures upon reentry into certain atmospheric layers.
Accordingly, such radome or window should possess the following properties: (a) high microwave transmission efficiency at or above temperatures of 2000.degree. C.; (b) resistance to dust, particle and rain erosion; (c) ablate only slightly at 2500.degree. C.; (d) a high temperature transmission loss of less than three decibels at 2350.degree. C.; (e) resistance to stress failure induced by large thermal differentials; and (f) readily machinable.
The earliest constructions for radomes have utilized ceramic type materials in monolithic, sandwich or multilayer constructions. Glass or glass fibers in combination with layers of foam or thermoplastics have been utilized as illustrated in U.S. Pat. Nos. 3,002,190 and 3,616,140. This type of construction loses strength and microwave transparency at high temperatures. Monolithic or sandwich wall constructions of alumina, fused silica or mullite have been fabricated and used. Alumina is very hard and resistant to rain erosion but is brittle and fails due to thermal stress and has a relatively high dielectric constant which promotes poor radar transmission (see U.S. Pat. Nos. 3,292,544 and 3,396,396). Monolithic radomes of mullite or Pyroceram 9606.RTM. have been fabricated, and while these are satisfactory for supersonic flight, they will fail due to thermal stress or rain erosion at velocities over about Mach 5. Monolithic silica material exhibits satisfactory radar transparency at high temperatures having a relatively low dielectric constant of 3.36. However, such silica material has poor rain erosion and particle impact resistance, is difficult to machine and, most importantly, exhibits excessive ablation at temperatures encountered by reentry vehicles.
Radomes or antenna windows fabricated as monolithic or multilayered structures have been made from nitrides including silicon nitride, boron nitride and silicon nitride layered with silica foam or boron nitride. Monolithic boron nitride or silicon nitride materials are unsatisfactory because they become opaque to radar at high temperatures. Even at lower temperatures the hexagonal form of boron nitride is insufficiently strong. In the cubic form, boron nitride cannot be made in sizes necessary for radomes. When boron nitride is combined with silicon nitride, the problem of high temperature radar transmission still remains an unsolved problem. A multilayer design consisting of silicon nitride and silica has poor high temperature radar transmissibility (see U.S. Pat. No. 4,358,772).
What is needed is a monolithic radar window material that can possess all the desirable physical characteristics outlined earlier above.
Therefore, it is an object of this invention is to provide a method of making a microwave radar transparent window material operable at temperatures above 2000 .degree. C., and which material possesses high tensile strength, is resistant to erosion as well as particle impact at such temperatures and at room temperature, and is highly machinable.